There are disclosed herein hole blocking layers, and more specifically, photoconductors containing a hole blocking layer or undercoat layer (UCL) comprised, for example, of a metal oxide, such as TiO2, dispersed in a mixture of a urea formaldehyde resin and a phenolic formaldehyde resin, and which layer is coated or deposited on a first layer like a supporting substrate and/or a ground plane layer of, for example, aluminum, titanium, zirconium, gold, or a gold containing compound.
In embodiments, photoconductors comprised of the disclosed hole blocking or undercoat layer enables, for example, the blocking of or minimization of the movement of holes or positive charges generated from the ground plane layer, and excellent cyclic stability, and thus color print stability especially for xerographic generated color copies. Excellent cyclic stability of the photoconductor refers, for example, to almost no or minimal change in a generated known photoinduced discharge curve (PIDC), especially no or minimal residual potential cycle up after a number of charge/discharge cycles of the photoconductor, for example about 200 kilocycles, or xerographic prints of, for example, from about 75 to about 250 kiloprints. Excellent color print stability refers, for example, to substantially no or minimal change in solid area density, especially in 50 to 60 percent halftone prints, and no or minimal random color variability from print to print after a number of xerographic prints, for example 70 kiloprints.
Further, in embodiments the photoconductors disclosed permit the minimization or substantial elimination of undesirable ghosting on developed images, such as xerographic images, including minimal ghosting, especially as compared to a similar photoconductor where the resin mixture disclosed herein is absent and at various relative humidities; excellent cyclic and stable electrical properties; acceptable charge deficient spots (CDS); and compatibility with the photogenerating and charge transport resin binders, such as polycarbonates. Charge blocking layer and hole blocking layer are generally used interchangeably with the phrase “undercoat layer”.
The need for excellent print quality in xerographic systems is of value, especially with the advent of color. Common print quality issues can be dependent on the components of the undercoat layer (UCL). When the undercoat layer is too thin, then incomplete coverage of the substrate may sometimes result due to wetting problems on localized unclean substrate surface areas. This incomplete coverage may produce pin holes which can, in turn, produce print defects such as charge deficient spots (CDS) and bias charge roll (BCR) leakage breakdown. Other problems include image “ghosting” resulting from, it is believed, the accumulation of charge somewhere in the photoreceptor. Removing trapped electrons and holes residing in the imaging members is a factor in preventing ghosting. During the exposure and development stages of xerographic cycles, the trapped electrons are mainly at or near the interface between the charge generation layer (CGL) and the undercoat layer (UCL), and holes are present mainly at or near the interface between the charge generation layer and the charge transport layer (CTL). The trapped charges can migrate according to the electric field during the transfer stage where the electrons can move from the interface of CGL/UCL to CTL/CGL, or the holes from CTL/CGL to CGL/UCL, and become deep traps that are no longer mobile. Consequently, when a sequential image is printed, the accumulated charge results in image density changes in the current printed image that reveals the previously printed image. Thus, there is a need to minimize or eliminate charge accumulation in photoreceptors without sacrificing the desired thickness of the undercoat layer, and a need for permitting the UCL to properly adhere to the other photoconductive layers, such as the photogenerating layer, for extended time periods, such as for example, about 750,000 simulated xerographic imaging cycles. Thus, a number of conventional materials used for the undercoat or blocking layer possess a number of disadvantages resulting in adverse print quality characteristics. For example, ghosting, charge deficient spots, and bias charge roll leakage breakdown are problems that commonly occur and which problems are minimized with the photoconductors illustrated herein.
Thick undercoat layers are sometimes desirable for xerographic photoconductors as such layers permit photoconductor life extension and carbon fiber resistance. Furthermore, thicker undercoat layers permit the use of economical substrates in the photoreceptors. Examples of thick undercoat layers are disclosed in U.S. Pat. No. 7,312,007, however, due primarily to insufficient electron conductivity in dry and cold environments, the residual potential in conditions, such as 10 percent relative humidity and 70° F., can be high when the undercoat layer is thicker than about 15 microns, and moreover, the adhesion of the UCL may be poor, disadvantages avoided or minimized with the UCL of the present disclosure.
Also included within the scope of the present disclosure are methods of imaging and printing with the photoconductive devices illustrated herein. These methods generally involve the formation of an electrostatic latent image on the imaging member, followed by developing the image with a toner composition comprised, for example, of a thermoplastic resin, colorant, such as pigment, charge additive, and surface additives, reference U.S. Pat. Nos. 4,560,635; 4,298,697 and 4,338,390, the disclosures of which are totally incorporated herein by reference, subsequently transferring the image to a suitable substrate, and permanently affixing the image thereto. In those environments wherein the device is to be used in a printing mode, the imaging method involves the same operation with the exception that exposure can be accomplished with a laser device or image bar. More specifically, the imaging members, photoconductor drums, and flexible belts disclosed herein can be selected for the Xerox Corporation iGEN3® machines that generate with some versions over 100 copies per minute. Processes of imaging, especially xerographic imaging and printing, including digital, and/or high speed color printing, are thus encompassed by the present disclosure.
The photoconductors disclosed herein are, in embodiments, sensitive in the wavelength region of, for example, from about 400 to about 900 nanometers, and in particular from about 650 to about 850 nanometers, thus diode lasers can be selected as the light source.